I wanted to use an Advance Centium ballast for some T5 lamps, but I can’t seem to find many 2x f8t5 ballasts for cheap. Would I be able to use the 1x14 watt t5 configuration on the centium and put the two F8 bulbs in series? Don’t really know enough about these lamps to hack around with them.

Connecting two F8T5 lamps to a F14 ballast would overload noth the lamps (F14 needs 165mA, F8 145mA at HF, so the F8 would run at about 9..10W), the ballast will then operate at higher voltage, so deliver 18..20W (unless it is also rated for the F21T5HE). Assuming really constant current behavior of the ballast, the T5HE ones use to be pretty close to that...

So it would be really bad?

If it helps the model is the ICN-2S28 one.

So it would be really bad?

It is a gamble. The ballast may happen to not be so strict current source and sag the current when the voltage goes higher, then the question is how much.
Or that overloading will truly happen and it will cause both lamps and ballast to age way faster than it is supposed to.

Or it will trigger the lamp EOL protection and so don't light at all (or it will refuse to operate after just partial lamp wear).

It does have a F21 option on the ballast, what is HE? What’s the difference? What stinks is that I only have space for a 12 inch lamp, don’t really have room for a larger one. The wattage doesn’t matter but space is tight. Any other options except buying a new ballast?

Well, I bet the ballast will run a couple of F8T5, but with some overdrive. If you don't mind a bit of lamps lifetime reduction you probably will be OK. Datasheet for ICN-2S28 lists running single 21W HE tube at a ballast factor of ~1.06 likely meaning lamp current will be slightly above of T5 HE tubes prescribed 170mA.  T5 HE series means the tubes are more or less descendants of historical F4-F13T5 tubes, just a bit optimized for modern tech, and running at about the same current density, but usually on HF.

Note that old classic T5's also have somewhat higher cathode resistance than modern T5 HE's, but that's *usually* OK.

It is always a big P.I.T.A to measure an actual power of a lamp running on HF, but you always can measure the system power at the ballast input and subtract the expected ballast loss of about 10%.

In my experience, T5 HE ballasts run classic T5 tubes rather happily, though I have not tried anything other than some short runs.

Ok I see. Is it a major loss of life? Or just a 1000 or so hours? Is there a better lamp I could be using? These are Chinese lamps, not some high quality so they probably wouldn’t last as long anyways. Thanks for the help, sorry for all the questions.

Also, an unexpected twist to consider, this ballast is capable of running a couple of 150V 28W T5HE tubes in series, so sure it has a greatly excessive OCV for running a couple of F8T5, just ~114V in the sum both. This is OK in a normal run, but expect a nice firework at EOL, as then the ballast will attempt to pump >50W in the failed tube end until the filament blows or the glass cracks

 

That sounds bad. Is there a better option?

Ok I see. Is it a major loss of life? Or just a 1000 or so hours? Is there a better lamp I could be using? These are Chinese lamps, not some high quality so they probably wouldn’t last as long anyways. Thanks for the help, sorry for all the questions.

Well, no one will tell exactly. Usually, fluorescents are relatively tolerant to overcurrent, unlike incandescents, as cathode spots just grow a bit bigger, and lifetime drops linearly proportional to overcurrent. BUT you are dealing with recent orient imports, and can expect thin filaments, poor quality activation, so the tube may run on the edge even at nominal current of 145mA. 

Man that stinks. What about a 3rd tube? Would that offset the  lifetime problems or Eol? I don’t care if they die early but I don’t want firey flourescent tube raining on a carpeted area.

That sounds bad. Is there a better option?

Just do not run the tubes into the EOL.

You may try to hunt down a ballast intended to run a couple of 4-pin 7-11W PL tubes. These are quite closely equivalent to old school T5's.

Man that stinks. What about a 3rd tube? Would that offset the  lifetime problems or Eol? I don’t care if they die early but I don’t want firey flourescent tube raining on a carpeted area.

Three and even four and probably five (!) tubes will work, but you will not have a preheat for the extra tube ends. You can leave this as is (as Philips Certalume series ballasts shamelessly do) but it will drop the lifetime even further!

Just do not force tubes into EOL and stop the thing when they form some dense sputtered 'socks'.

Thanks for the help and warning. The one time I actually buy the parts ahead of time and think it will probably be fine is the one where there doesn’t seem to be a safe solution. Too bad. Thanks though, if there are any more hacks let me know. What a shame. Maybe I could get some tube protectors, I am always checking on my fluorescents anyways.